There has been proposed a foreign object sensor, for example a raindrop sensor, wherein a prism, a light emitting unit (light source), and a light receiving unit are disposed, inside of a windshield or windowglass of a vehicle. The light impinging on the windowglass from the light emitting unit through the prism is totally reflected at a boundary surface between the windowglass and the outside air and the reflected light is received by the light receiving unit. If there is a raindrop on the outer surface of the windowglass, a part of the light impinging on the windowglass escapes though the raindrop so that the quantity of reflected light, that is the quantity of light received by the light receiving unit, decreases. This causes a change in the output of the light receiving unit indicating the presence of a raindrop.
There is a problem in the raindrop sensor described above because not only is it required to provide a complex processing circuit for eliminating disturbance by external light, for example sunlight, impinging on the light receiving unit through the prism from the outside of the windowglass, but it also may become impossible to perform raindrop detection because the light receiving unit is saturated by the external light.